Allow public viewing of profiles and people calendars

Summary: Ref T4830, this allows profile view and calendar profile view to be publicly visible. Also adds a note reminding the user of visibility.

Test Plan: Log out, visit a profile and various links.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T4830

Differential Revision: https://secure.phabricator.com/D13676